1. Field of the Invention
The present invention relates to novel, soft propylene-based resin compositions.
2. Discussion of the Background
Soft polyvinyl chloride has heretofore been widely used as a type of soft resin that has flexibility, transparency and heat resistance. However, since polyvinyl chloride produces harmful substances when incinerated, it is desired to develop a novel soft resin which can be substituted for soft polyvinyl chloride. Considering the production costs of resins, an olefin-based soft resin may be one candidate for it.
For example, it is known that ethylene-based resins prepared by copolymerization of ethylene with an α-olefin to have a lower degree of crystallinity have good flexibility and transparency. However, when the content of the comonomer, α-olefin is increased in the ethylene-based resin of this type in order to improve the flexibility and the transparency of the resin, the heat resistance of the resin is often lowered. Accordingly, it is extremely difficult to improve all three properties of flexibility, transparency, and heat resistance of ethylene-based resin.
On the other hand, atactic polypropylene is known as a type of propylene-based soft resin, which is produced by the use of a Ziegler-Natta catalyst or a metallocene catalyst. Of those, the atactic polypropylene that is produced by the use of a Ziegler-Natta catalyst has a broad molecular weight distribution and a broad stereospecificity distribution, and the amount of the sticky component in the polymer is large. Therefore, the polymer is difficult to apply to films and sheets. The other atactic polypropylene that is produced by the use of a metallocene catalyst can be specifically controlled to have a narrowed molecular weight distribution and a narrowed stereospecificity distribution, but its heat resistance is poor since atactic polypropylene is naturally an amorphous polymer. Accordingly, these types of atactic polypropylene also can not satisfy all the requirements of improved flexibility, transparency, and heat resistance, like the above-mentioned ethylene-based copolymer.
On the other hand, stereoblock-structured polypropylene that has isotactic and atactic segments is known. When the isotactic segment is increased in these polymers, then the crystallinity of the polymer is high and therefore the flexibility thereof is poor; and when the isotactic segment in this is reduced, the flexibility and the transparency of the polymer may be high, but the heat resistance thereof is poor and the polymer becomes significantly sticky. Accordingly, the polymer is defective in these points, and it can not satisfy all the requirements of improved flexibility, transparency, and heat resistance. In addition, since the known, stereoblock-structured polypropylene is extremely sticky as so mentioned above, it is generally produced through polymerization in an inert hydrocarbon solvent.
Further, a soft polypropylene-based resin composition that contains atactic polypropylene and an ethylene/propylene copolymer is now under investigation. However, both the atactic polypropylene and the ethylene/propylene copolymer to constitute the resin composition do not exhibit a melting point, or may exhibit a low melting point, and therefore, the resin composition comprising the two can not be very resistant to heat like the above-mentioned atactic polypropylene. To that effect, the resin composition could not satisfy all the requirements of improved flexibility, transparency, and heat resistance.
Thus, there remains for substitutes for soft polyvinyl chloride which do not suffer from the above-mentioned drawbacks,